1. Field of the Invention
This invention relates to a method of making a magnetic recording material, more particularly, to a method of making a magnetic recording material having a ferromagnetic metal thin layer, such as a magnetic tape, by a plating method.
2. Description of the Prior Art
It is known that ferromagnetic metal thin layers can be used for a magnetic recording materials. Firstly, because a thin magnetic recording layer provides a high saturated magnetic flux density, and, secondly, while a ferromagnetic metal thin layer can be used for magnetic recording in general, it is especially useful for high density magnetic recording due to its capability of exhibiting a very low to a very high coercive force.
As uses of ferromagnetic metal thin layers, it is known that a ferromagnetic metal thin layer having a low coercive force; i.e., a soft ferromagnetic metal thin layer, can be used as a core matrix memory element and an alternating current memory element, while a ferromagnetic metal thin layer having a high coercive force, i.e., a hard ferromagnetic metal thin layer, can be used as to a medium for magnetic recording as disclosed in, for example Soshin Chikazumi "Kyojiseitai no Butsuri" (Physics of Ferromagnetism) pages 327 to 330, published by Shyokabo Co., Ltd. Ferromagnetic metal thin layers are also useful for photomagnetic recording using a photomagnetic effect e.g., the Kerr magnetic effect, Faraday rotation, etc.
Further, it is known in the use of ferromagnetic metal thin layers that more effects appear when the ferromagnetic metal thin layer exhibits magnetic anisotropy. With respect to the orientation of such magnetic recording materials, this has been studied and is disclosed in U.S. Pat. Nos. 1,949,840; 2,418,479, Japanese Patent Publications 3427/1957; 21,158/1964, etc.
In order to obtain a ferromagnetic metal thin layer exhibiting magnetic anisotropy, many methods in which a web support is plated in a magnetic field have been proposed. The inventors of the present invention have also studied methods of making a ferromagnetic metal thin layer which is suitable for magnetic recording materials having a high coercive force, especially with respect to methods of applying uniaxial anisotropy to the ferromagnetic metal thin layer, and they made it clear in Japanese Patent Application (OPI) 15999/1974 that in order to obtain a ferromagnetic metal thin layer exhibiting uniaxial anisotropy, a magnetic field of a definite direction need not be applied during the total plating time, but need only be applied for a certain period after the beginning of plating.
The degree of orientation R is expressed as R = (SQ// - SQ.perp.)/(SQ// + SQ.perp.). SQ is the ratio of the maximum magnetic flux density Bm to the residual magnetic flux density Br: Br/Bm, where the squareness ratio SQ// is measured along the axis of the easy magnetization direction and the squareness ratio SQ.perp. is measured at right angles to the axis of the easy magnetization direction. When a magnetic field as shown in FIG. 1 is applied during a plating process, R has the relationship as shown in FIG. 2.
In more detail, assuming that the time of finishing of magnetic field application t.sub.2 is the time of finishing plating, the relationship of R to the time of beginning the magnetic field application t.sub.1 is shown as a t.sub.1 -R curve. The more t.sub.1 increases, the more the period of magnetic field application and the R value decreases. On the other hand, assuming that t.sub.1 is the time of beginning plating, the relationship of R to t.sub.2 is shown as a t.sub.2 -R curve. The more t.sub.2 decreases, the more the period of magnetic field application and the R value decreases. It can be understood by comparing these two curves that the saturated R values of both curves are naturally equal to each other, and the time t.sub.0 which indicates the half saturated R value of both curves are almost equal each other, too.
Expanding somewhat upon the above, assume that t.sub.1 is the time between the time of beginning plating and the time of beginning magnetic field application and t.sub.2 is the time between completing magnetic field application and completing plating. First, assume the time of completing plating is fixed at t.sub.2 ; the case of changing t.sub.1 will be discussed. In this case, the t.sub.1 -R curve shows the changes in R. When t.sub.1 is small, R is large since the period of orientation will be increased. On the other hand, when t.sub.1 is large, R will be small since the period of orientation is decreased (when t.sub.1 = t.sub.2, R = 0). Second, assume the time of beginning plating is fixed at t.sub.1 ; the case of changing t.sub.2 will be discussed. In this case, the t.sub.2 -R curve shows changes in R. If magnetic field orientation is halted a long period of time before the termination of plating, R will be small since the period of orientation is decreased. On the other hand, if t.sub.2 approaches the time of completing plating, R will be large since the period of orientation is increased. When t.sub.1 is small (magnetic orientation begins very shortly after plating) and t.sub.2 is large (magnetic orientation ends very close to the end of plating), then the orientation time will be long, and, accordingly, R approaches the saturation value.
As one skilled in the art will appreciate, the squareness ratio of a magnetic recording material shows the difference between the orientation direction and a direction perpendicular to the direction of orientation. The greater the orientation value, the greater the difference between SQ // and SQ.perp., and the more R is increased. With reference to the t.sub.1 -R curve, one can select an appropriate time on the t.sub.1 -R curve which brings the R value to 90% of its saturated value, i.e., the time of beginning magnetic field application t.sub.a can be obtained. In a similar manner, with reference to the t.sub.2 -R curve, the time of completing magnetic field application t.sub.b can be obtained.
Based on the above, one skilled in the art can easily select a R value under the conditions t.sub.1 = t.sub.a, t.sub.2 = t.sub.b.
In FIG. 2, assuming that t.sub.2 is the time of finishing plating, t.sub.a corresponds to the value of t.sub.1 indicating the time when the R value becomes 90% of the saturated R value, t.sub.1 is the time of beginning plating, and t.sub.b corresponds to the value of t.sub.2 indicating the time when the R value becomes the 90% of the saturated R value, a ferromagnetic metal thin layer having an almost saturated uniaxial anisotropy can be obtained with good efficiency by a plating in a magnetic field if t.sub.a and t.sub.b are selected as the time of beginning magnetic field application and the time of finishing magnetic field application, respectively, i.e., t.sub.1 = t.sub.a and t.sub.2 = t.sub.b. Even if the period of magnetic field application is greatly reduced, the degree of orientation R which is 90% of the saturated value can be still obtained.
In this point, the degree of orientation R value of 90% of the saturated value is selected for t.sub.a and t.sub.b. As one skilled in the art will appreciate, an extremely long orientation time is required to increase R to its saturation value, i.e., large scale magnetic field apparatus is required in a continuous process as is contemplated in the present invention. Accordingly, orientation is finished at a level lower than the saturation value. If this level was too low as compared with the saturation value, product quality problems sometimes occur. However, no problems are encountered on a commercial scale when R is about 90% or more, and, considering the orientation period, from a process viewpoint a value of about 90% of saturation is quite economical. This value is not, however, to be construed as limitative upon the present invention since the exact product quality required will vary from user to user, and in certain instances R values higher than 90% of the saturated value may be required by an user without any particular reference to process economics.
Since magnetic recording materials are used in many different applications as described before, orientation must be conducted in any direction. For instance, a magnetic tape for magnetic video recording must be oriented diagonally to the long direction.
Prior art processes where a magnetic field is applied while plating cannot provide a magnetic axis of easy magnetization in any desired direction. Further, it is not preferred that abrasion and/or a scratches occur on a web during conveying a web with a roller. Furthermore, if a process in which a web is conveyed without any contact was used in order to remove the defects produced by conveying with a roller, it was impossible to uniformly provide an orientation effect on the surface of the web since the conveying without contact was unstable due to flapping and the like.